Communication cable filling compounds are employed, in general, as water barrier materials and, specifically, in applications involving fiber optic cables as both water barrier materials and as protective compounds. Formulations of cable fillings compounds accordingly are determined by application requirements, which can vary widely.
Fiber optic cables are increasingly employed in transmission of data and other communications. The fiber optic element, for protection against mechanical shock or bending since the fiber optic element is relatively fragile, is inserted into a protective casing. The space between the fiber optic element, or a series of such elements, is filled with a thixotropic composition which is a stable cushioning agent to protect the fiber optic elements but which does not deleteriously affect the optical qualities of the elements.
The required properties of the filling compound can be mutually exclusive. The compound is required to retain its softness at low temperature, -40.degree. C., and to be resistant to flow at high temperature, +80.degree. C. The compound is required to retain its softness at low temperatures, -40.degree. C., after exposure to a high temperature, 93.degree. C. (200.degree. F.), for a prolonged period. The filled communication cable containing the filling compound is required to be waterproof and gas tight. One method of filling the sheaths of these communication cables requires injecting the sheaths of these cables with the filling compound in a fluid state under pressure. In another method, the filling compound can be introduced under pressure into the cable sheath during manufacture and extrusion of the cable sheath. During the filling process, the filling compound penetrates the voids between the optic fibers and its cable sheathing under high pressure, creating high shearing stresses. The filling compound should retain its characteristics despite the shearing stresses by not exhibiting phase separation or change in viscosity. During use, the filling compound needs to be sufficiently fluid to avoid build-up of tensile or compressive forces upon the optic fibers to cause modification of the optic fiber's optical characteristics and yet be resistant to flow to ensure that bending of the cable will not cause voids within the cable sheathing.
The filling compound accordingly is required to have a low viscosity at room temperature to allow filling of cables during the cable manufacture and yet not demonstrate a hardness at low temperatures which will result in a mechanical stressing of the optic fibers due to increase of viscosity during use. Preferably, the filling compound exhibits softness at low temperatures and demonstrates no change in viscosity or phase separation after exposure to high temperatures up to 93.degree. C. for prolonged periods.
The requirement that the filling compound retain its softness at low temperature -40.degree. C. and yet be resistant to flow at high temperature +80.degree. C. conventionally has been treated in the prior art by employing a thickening agent to form a gel-like substance comprising a petroleum hydrocarbon or a silicone, or a polyglycol or a similar compound, and a thixotropic agent. The petroleum hydrocarbon and the thixotropic agent as a mixture comprises a non-flowing or nearly solid yet gel-like body while in a rest condition but the mixture will assume the properties of a fluid under mechanical stress, yet be resistant to flow at a high temperature.
However, the mutually exclusive requirements of the filling compound, that the filling compound retain its softness at low temperature and yet be resistant to flow at high temperature, have been difficult to satisfy because of the conflicting physical characteristics of the mixture components. A thickening agent is frequently utilized to control the flow rate at high temperatures. However, exposure of the filling compound to high temperatures can affect the resultant softness at low temperature by causing the filling compound to irreversibly became more brittle and hard at the low temperature.
The petroleum hydrocarbon conventionally has been selected from the group consisting of a petroleum jelly, a liquid polybutene, a hydrocarbon oil with a low aromatic content, or an aliphatic and an aromatic hydrocarbon oil or mixtures thereof. The thixotropic agent typically is a non-hydrocarbon material a silicious material such as diatomaceous earth, colloidal silica, pyrogenic silica, silica aerogel and similar silica materials in a finely divided state.
The inclusion of silicious material in the filling composition as a thixotropic agent in the form of an extremely fine powder, because of the dispersion of the powder into the gel, can result in a hydrophilic medium subject to the entry of water and water vapor from punctures or breaks in the cable's outer sheathing.
Silicious materials in a finely-divided physical state of colloidal dimensions with a high surface area can demonstrate hydrophilic surface properties which can be detrimental to applications requiring a water barrier or water vapor barrier.
Although filling compounds for communication cables are primarily intended for filling the spaces within the communication cables, the filling compounds are used to inhibit penetration of water or water vapor, particularly buried cables which are subject to the ingress of water, or to entry and condensation of water vapor.
The protective casing of the fiber optic communication cables and/or wave guides are designed as water-proof shields enclosing the fiber optic elements. Water or water vapor can enter a cable through punctures or breaks in the cable's outer sheathing or wave guide. These breaks can occur as a result of mechanical damage to the protective casing or of initial defects incurred during production or laying of the cable. Once a break has occurred in the outer sheathing, the entry of water is permitted and allows water to flow along the cable and to mix with the filling compound.
The filling compound is also known to be used as a water blocking compound to inhibit the introduction of water into cables as a result of damage to the cable sheaths when cables are installed in ducts or directly buried in the ground, and to prevent such water from travelling along the interior from the point of entry.
Conventional petroleum gels have been relied upon as filling compounds with the addition of hydrocarbon micro-crystalline waxes and low molecular weight polyethylene resins to overcome water entry problems and to diminish migration and leakage from cables having a flaw. Petroleum gels have a tendency to seep from cable ends or from a flaw developed in the cable especially at elevated temperatures to which the cable might be subjected in installation or in use in warm environments. Inclusion of amounts of silicious material in the petroleum gel has been found to result in a filling composition in which migration of the petroleum gel has been considerably reduced, but with lessened inhibition to water penetration.
The requirement that the filling compound retain its softness at low temperature -40.degree. C. and yet be resistant to flow at high temperature +80.degree. C. conventionally has been satisfied by use of a thickening agent to form a gel-like substance. The thickening agent typically has been a silicious material which, as noted above, may have hydrophilic characteristics.
The tendency for the components of a filling compound to separate because of poor compatibility, i.e., to demonstrate a degree of syneresis, can cause the filling compound to be unsuitable for applications which are typically at higher temperatures, such as +80.degree. C.
The tendency for the components of a filling compound to develop a heat history upon exposure to elevated temperatures, i.e, 93.degree. C. (200.degree. F.), for prolonged periods can cause the filling compound to be unsuitable for applications which are typically at low temperatures, i.e., at -40.degree. C., because of irreversible development of a more brittle and harder gel formation at the higher temperatures.
Two tests have been devised to determine the softness characteristics of a buffer tube filing compound at -40.degree. C. and at 80.degree. C. The first test comprises a cone penetration test at -40.degree. C., ASTM D937. The second test comprises a slump test at +80.degree. C. for a period of 24 hours which requires no separation of filling components and no loss of filling compound from a u-shaped channel lying on its side for the required period and temperature. A variation of the slump test comprises the requirements that no drip out flows from a phase separation from a vertically suspended tube over a period of 24 hours at +80.degree. C. This variation is termed a drip test. Drip out is defined as a drip from a phase of the separation.
An aging test of the filling compound has been devised to determine the softness characteristics of a buffer tube filling compound after exposure to an elevated temperature, 93.degree. C. (200.degree. F.) for 45 minutes. The compound is then allowed to cool to room temperature, +21.degree. C., and then is cooled to -40.degree. C. The cone penetration test, ASTM D937, is repeated at -40.degree. C. to determine softness characteristics of the compound after the heat treatment at 93.degree. C.
It is therefore an object of this invention to provide a thixotropic gel composition for filling communication cables containing fiber optic conductors which has an improved resistance and inhibition to penetration of water and water vapor, particularly buried cables which are subject to the ingress of water, or to entry and condensation of water vapor which can enter through punctures or breaks in the cable's outer sheathing or wave guide.
It is therefore an object of this invention to provide a thixotropic cable filling compound which retains its softness at low temperature, -40.degree. C. even after exposure to an elevated temperature, is resistant to flow and syneresis at temperatures to +80.degree. C. and the thixotropic cable filling compound has resistance to ingress of water and water vapor.